Basic Antenna Theory

Radio waves are generated by
electrons accelerating in the antenna.

Consider a transmitter perpendicular
to the ground. The electrons in the antenna, when a
signal is applied, are changing their velocities
continuously (i.e. moving up and down very quickly)
in response to the applied signal.

For a station that broadcasts at a
wavelength of 1500m, the antenna needs to be 750m
long. This is because there is a 'virtual antenna'
caused by the aerial being earthed in the ground:

The transmitting aerial (and the receiving aerial) need
only be half-the-wavelength tall.

Now if this transmitter has no
directional properties (i.e. it radiates in all
directions equally), it has a coverage area, assuming
completely flat ground, that is a perfect circle:

(View from above - antenna in centre; blue is
coverage area)

Broadcasters rarely use a non-directional aerial though.
It is possible to 'force' the energy radiated by the transmitter into
particular directions - the aerial becomes directional. Directional
aerials are used to great effect near the coasts of the UK, where the
broadcasters do not want their signal to be easily picked up on the
continent.

It is important that signal doesn't
leak onto the continent since i) continental
stations use the same frequencies and leaking signal
would cause interference and ii) some
programme broadcast rights apply to the UK only.

Under unusual weather conditions,
despite the best efforts of broadcasters both in this
country and abroad, signals travel much further than
they normally would and interfere with reception of
stations using the same channel, causing
co-channel intereference.

Let's switch the emphasis from the
transmitting aerial to the receiving aerial. Similar
principles apply for receiving aerials as for
transmitters as above.

The Half-Wave Dipole

There is only one part of a receiving aerial that is active,
i.e. does the receiving and is connected to the TV/radio set. This active
element is called the dipole. The simplest design of antenna
would consist of a dipole only:

A half-wave dipole

In the diagram above, there are two wires marked 'to receiver.'
For UHF and VHF, one wire will be the copper-core and the other the
copper braiding of a co-axial cable.

Before we proceed, a quick word about
gain. Although having a technical definition,
for us 'gain' can mean "the effectiveness with which
a receiving aerial receives a signal."

The diagram below shows the reception
pattern of a half-wave dipole. The blue area is where
the gain is higher than a certain value; the dipole
is in the centre:

We can change the directivity of the
aerial by adding other elements. Any other
elements that we add to the basic half-wave dipole
are called passive elements and are not
connected electrically to the dipole.

There are two types of passive
elements:

Directors

Directors alter the directivity of
the aerial so that the aerial's gain is improved in
front of the dipole. Most aerials have more than one
director, and the more directors the aerial has the
better the aerial is at picking out the signal from
the required source and rejecting signals from other
angles.

These diagrams do not show the cross-bar that holds all
the elements in place as it does not much affect the characteristics
of the aerial.

The spacing between the directors,
diameter of the tubing used and the spacing between
the first director and the dipole are important in
practice but will be disregarded here. The length of
the directors governs the bandwidth of the aerial
(over which channels it is effective), but suffice it
to say that it is about 75% the length of the
dipole.

The gain of the dipole with directors
in place looks like this:

Notice how the gain is now more
focused in the direction of the directors.

As stated earlier, the more directors
an aerial has the more focused the gain is in the
direction of the directors. Every new director added
becomes less effective though, and in practice it is
only worth adding 18-20 directors to the aerial, as
any more than this wouldn't increase the gain very
much.

On the diagram above, the aerial
still has some gain at the rear - in other words, it
can still receive signals from behind. This is known
as a low front-to-back ratio.

The Reflector

To improve the front-to-back ratio we
can add the second type of passive element, a
reflector. The reflector reflects signal coming in
from the back of the aerial whilst improving the
forward gain.

This design is called a Yagi-Uda
array, after its creators.

Again, the length, size and position
of the reflector affect the aerial's properties, but
we won't go into that here.

The reflector can take the shape of a
metal plate (with holes in it, making the aerial more
impervious to wind) or several rods spaced
equidistant from the centre of the dipole.

The result is that there is less gain
behind the aerial and more, where we want it to be,
in front:

Folded Dipoles

In order to minimise signal loss it is important that
the impedance (a sort of resistance for AC) of the dipole matches
that of the feeder cable and the receiving set.

The impedance for the type of dipole discussed above is
about 75 ohms. More often than not though the impedance needs to be
altered to match the cable and receiving set characteristics.

This change of impedance is acheived
by folding a rod over so that its folded length is
still half-a-wavelength:

Now we know what each constituent
part of an aerial is called and what its function is,
let's look at some examples in the field.